The invention relates to a folded fin heat sink assembly and to a method of fabricating a folded fin heat sink assembly for use as a cooling solution in micro-electronics and/or telecommunication applications. In particular, the invention relates to a method of mounting and soldering an aluminum folded-fin assembly to a copper base plate to form a folded fin heat sink assembly.
Integrated circuit devices are increasingly being used in modern electronic applications such as computers. During normal operation, integrated circuit devices generate significant amounts of heat. If this heat is not continuously removed, the device may overheat resulting in damage to the device and/or a reduction in operating performance. One economic solution for cooling such devices is a heat sink mounted on top of the integrated circuit device. Due to this contact, heat generated by the integrated circuit is conducted into the heat sink and away from the integrated circuit. A fan may be provided to assist in moving the heat away. Also, as a general rule, the performance of integrated circuit devices is likely to improve when they are operated at lower temperatures. Hence, heat sink solutions which facilitate a lower integrated circuit operating temperature have an economic value over heat sink solutions offering higher integrated circuit operating temperatures.
Over the years, there has been a trend toward reduction in size and increased clock frequency speeds of integrated circuit devices, coupled with increases in the number of transistors and therefore capacitance within the integrated circuit. These trends have resulted in a proportional increase in the power used by the integrated circuit. Consequently, the heat generated by these devices has also increased. In order to adequately cool these high powered integrated circuit devices, heat sinks with greater cooling capacities have evolved.
Also, the trend toward reduction in size inherently results in greater heat flux density being imposed onto the heat sink. The efficiency of the heat sink in spreading the heat from the heat source to the fins can become a limitation of the heat sink design.
Historically within the microprocessor industry, the majority of heat sink solutions have used aluminum extrusions. In aluminum extrusions, surface area aspect ratios are typically limited to a maximum ratio of 12:1.
In today""s marketplace, with microprocessor solutions being offered in the 2.2 GHz clock frequency range, cooling requirements often cannot be met by the technical capabilities offered by aluminum extrusion technology. In order to meet this need heat sink designs have evolved from simple one piece aluminum extrusions to an assembled heat sink format with two or more components in which a denser plurality of fins formed by folded-fin technology is used. Assembled heat sinks offer the ability to use higher conductive materials such as copper in the design of the base plate along with aluminum fins, when spreading the heat from the heat source becomes a limitation. Folded-fin technology, with its low thickness range (0.004xe2x80x3-0.040xe2x80x3) and tight fin density capabilities offer heat sink aspect ratios which can approach 40:1 and correspondingly larger surface areas for heat dissipation.
A typical folded-fin heat sink assembly comprises a base plate and a folded-fin assembly mounted an top of the base plate, the folded-fin assembly having a plurality of joined folded-fins extending upwardly from the base plate. A shroud may also be provided surrounding a substantial portion of the folded-fin assembly. The folded-fin assembly is produced by feeding strip aluminum or copper material through a met of blades which are actuated through cam action to produce its accordion-like structure, having a lower web, an upwardly extending fin portion, an upper web, a downwardly extending in portion, and so on, repeating in a progressive zig-zag fashion.
Typically the base plate and the folded-fin assembly are made of materials which have a high thermal conductivity; materials such as aluminum (approximately 200 W/mK) or copper (approximately 400 W/mK) and, in some cases, these two components comprise the heat sink in its totality.
The conventional method of assembling aluminum fins to a copper base plate uses a thermal epoxy process to assemble the dissimilar materials together. This however reduces the performance of the heat sink, as thermal epoxies in general have conductivity levels of only about 1 W/mK.
An alternative approach involves nickel plating the aluminum fins so that they can then be soldered to the copper base plate, using generic tin/lead (Sn/Pb) solder, it being noted that Sn/Pb solder will not bond aluminum. However, nickel plating densely packed aluminum fins typically requires an electro-less nickel-plating process, which adds significant cost in manufacturing.
Yet a further approach involves a brazing process for bonding the aluminum fins to the copper base plate. However, this process step requires an intermediary material such as nickel or iron between the aluminum fins and copper base plate to ensure that low temperature inter-metallics of Cu and Al do not form, resulting in an uncontrollable joining process. Once again, this process requires the additional manufacturing cost associated with providing an intermediary material.
It is therefore desirable to provide a process for bonding the aluminum fins to the copper base plate in which low thermal resistance levels can be achieved within the joint encompassing the bonded elements by utilizing good solder joint practices such as i) the soldering of a flexible (folded fin) element to an inflexible (copper plate) element in order to provide for good flatness control, and ii) providing regular gaps between adjacent elements to enable the even flow of excess solder in the liquidus state, such that the extent of solder material between the elements can be minimized for a superior thermal conduction path, and iii) provide a healthy meniscus curve solder joint between gaps to provide structural strength and facilitate enhanced thermal conductive paths to the individual fin elemental.
Given that there is also a preference in certain applications that the copper base plate be nickel plated, it is also desirable that the process be effective for bonding the aluminum fins to either an unplated base plate or a nickel plated copper base plate.
It is an object of the invention to provide a method of bonding an aluminum folded-fin assembly to an unplated or a nickel plated copper base plate, to form a folded-fin heat-sink assembly, which obviates or mitigates the disadvantages of known methods and processes as discussed above.
In a first aspect, the invention provides a method of joining an aluminum folded fin assembly to a copper base plate to form a folded fin heat sink assembly, comprising the steps of:
(a) placing a sheet or paste of Snxe2x80x94Zn solder upon the base plate,
(b) placing the folded fin assembly on the solder sheet or paste,
(c) heating the base plate, folded fin assembly and solder to a temperature exceeding the liquidus temperature of the solder and allowing the solder to flow, and
(d) cooling the solder to form a soldered joint between said base plate and the folded fin assembly.
When the solder is in the form of a sheet, the method may further comprise applying a flux, preferably to the base plate and to the fins, or alternatively to the upper and lower surfaces of the sheet between steps (a) and (b). Alternatively, the solder may be in the form of a paste which includes a flux for the solder. The paste may be stencilled or screen printed onto the base plate.
Preferably, the solder comprises about 91% Sn and about 9% Zn.
The base plate and folded fin assembly may be secured in an assembly fixture and retained therein during steps (b), (c) and (d). Alternatively, the base plate may be placed in a temporary assembly fixture prior to step (a); following step (b), individual fins of the heat sink and the base plate are laser tack welded together at multiple points; the base plate is removed from the temporary assembly fixture; and the base plate, the folded fin assembly and the solder are heated in step (c) and cooled in step (d).
Step (c) may be performed in a reflow oven or at a hot plate soldering station.
According to a further aspect of the invention, there is provided a folded fin heat sink assembly comprising one or more aluminum folded fin assemblies, each of which is soldered to a copper base plate by means of an Snxe2x80x94Zn solder, preferably having about 91% Sn and about 9% Zn.